1. Field of the Invention
The present invention relates to a lithium soap lubricating grease containing, in addition to the usual lubricating grease base and the customary lubricating grease additives, a thickening agent consisting of a lithium soap, preferably a lithium soap of a hydroxystearic acid and of an alkali borate.
2. Description of the Prior Art
In many applications, lubricating greases are superior to lubricating oils. They are employed, for example, to prevent the infiltration of harmful foreign matter by providing a barrier of grease, for example, in antifriction bearings or stuffing boxes. Greases are also employed to guarantee a very low need of lubricants, for example, on axle heads, in bearings with intermittent operation. Also, greases may be used to keep maintenance expenses low, for example, where bearing accessibility is difficult or in household appliance bearings. Greases are also preferred at points of lubrication with slow sliding movements, high pressures per unit area, and/or considerably varying loads.
Several different lubricating greases are in wide use, especially aluminum soap greases, calcium soap greases, sodium soap greases and lithium soap greases. Lithium soap greases have been employed in large scale for many years. The widely spread use of said lithium soap greases follows as a consequence of their combining all the essential advantages of calcium and sodium soap greases, i.e., they have excellent water-resistance, a low consistency at low temperatures, and upper service temperatures of above 100.degree. C.
German Offenlegungsschrift 2 157 207 disclosed a process for producing lithium soap grease. The thickening system, necessary in addition to the lubricating oil, revealed 3 main components:
A lithium soap of a hydroxy-fatty acid having from 12 to 24 carbon atoms; PA1 A lithium salt of a second hydroxy-fatty acid having from 3 to 14 carbon atoms; and PA1 The dilithium salt of a dicarboxylic acid having from 4 to 12 carbon atoms, and the dicarboxylic acid may, optionally, be replaced by monolithium salt of boric acid. PA1 a. employment of a long-chain hydroxy-fatty acid which is present as lithium salt in the final mixture, and PA1 b. incorporating minor amounts of alkali borate, preferably of a lithium tetraborate, directly into the said lubricating grease mixture as opposed to forming the alkali borate in situ during saponification. PA1 Liquid phase -- base oil -- PA1 Thickener PA1 Additives
It has been shown, however, that in many cases the temperature stability of these greases is not sufficient, e.g., in lubricating the wheel bearings of some types of passenger cars. In cars equipped with disc brakes near the wheel bearings, bearing temperatures of from 180.degree.-200.degree. C were encountered while driving on mountain roads which required frequent braking. By using lithium soaps of the prior art, a risk of disk brake failure is run which is caused by the leakage of the grease out of the bearings.
Lubrication of hot air ventilator bearings is another example. Here, too, temperatures of from 150.degree. to 200.degree. C may be reached in the bearings. Lithium soap greases having a drop point of up to 200.degree. C can no longer be used for lubricating these bearings.
U.S. Pat. No. 3,758,407 discloses a grease which is made by mixing together a fatty acid, lithium hydroxide and boric acid with heat. The resultant grease, however, possesses a dropping point below acceptable levels.
It is an object of this invention to produce a grease having a dropping point higher than has heretofore been possible.